Mark overridden destructors with 'override' and remove 'virtual'

This silences a new warning in clang 5.0

Change-Id: Ieb5b75a6ffed60107c3fd16075d2ecfd515b55e8
Reviewed-on: https://skia-review.googlesource.com/10006
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/src/pdf/SkPDFDocument.h b/src/pdf/SkPDFDocument.h
index 15e1479..aa6dcb2 100644
--- a/src/pdf/SkPDFDocument.h
+++ b/src/pdf/SkPDFDocument.h
@@ -50,7 +50,7 @@
                   const SkDocument::PDFMetadata&,
                   sk_sp<SkPixelSerializer>,
                   bool);
-    virtual ~SkPDFDocument();
+    ~SkPDFDocument() override;
     SkCanvas* onBeginPage(SkScalar, SkScalar, const SkRect&) override;
     void onEndPage() override;
     void onClose(SkWStream*) override;